1. Field of the Invention
The present invention relates in general to a method for recovering a usable alloy from scrap or secondary materials. It more specifically relates to a method for obtaining a titanium alloy by refining a charge of titanium scrap in an electron beam furnace.
2. Description of the Prior Art
Titanium ingots are generally obtained by consumable-electrode arc melting a pressed electrode. For example there is disclosed in U.S. Pat. No. 2,686,822, issued to R. E. Evans et al, a pressed consumable electrode comprising granular titanium admixed with various alloying elements. Evans et al melt the consumable electrode in a gas tight furnace containing an inert atmosphere; however, remelting can also be accomplished in a vacuum environment.
Double melting is usually employed to insure homogeneity. The use of vacuum melting for the production of ingots eliminates impurities found in sponge titanium such as hydrogen and magnesium chloride. Therefore the highest quality titanium ingots are obtained from consumable-electrode vacuummelted ingots. As used herein titanium sponge is that product obtained by the reduction of titanium tetrachloride by magnesium or sodium.
Because of titanium's density of about 0.16 pounds per cubic inch (compared with steel at 0.28) and the strength of titanium alloys at elevated temperatures the largest consumer of titanium for structural applications has been the aerospace and aircraft industry. For example, extensive use of titanium alloy forgings and bars are found in air frames and jet engine construction. In the construction of rocket nozzles and large diameter rings for jet engines, stringent quality requirements must be met. Therefore titanium starting stock must be in a very pure state. For this reason cast consumable-electrode vacuum melted ingots are employed. The cast ingot is converted into a billet by any conventional means such as by forging. The forged billet is thermomechanically worked, heat treated and machined to final dimensions.
A forged billet weighing approximately 900 pounds will produce a finishsed large diameter ring weighing approximately 150 pounds. When the solid billet is converted into a ring-shaped blank, there is substantial material loss in the form of punch-outs, croppings, scale and machine turnings. Since the larger pieces such as end croppings and punch-outs are relatively uncontaminated they can be returned to the ingot producer and remelted as prime charge materials.
The largest material loss is in the form of machine turnings and chips. Utilization of scrap in this form presents an entirely different problem. First of all the turnings and chips are contaminated with machine lubricants, scale and minute pieces of tool bits. Secondly, the analysis of the alloy has been altered. It is well known that titanium has a natural affinity for oxygen and nitrogen. During machining of the titanium stock the resultant chips and turnings become very hot. It is this combination of the elevated temperature and large surface area of the scrap that results in the titanium combining with atmospheric oxygen and nitrogen. Furthermore, the carbon content on the surface of the scrap is also increased as a result of the hot surface reacting with the cutting lubricant.
Some of this machine scrap can be reclaimed and reused by merely removing the machine oil and loose scale. For example, non-tin bearing titanium scrap reclaimed in this manner can be employed in the steel industry as an alloying addition. However, many titanium alloys containing tin as an alloying addition generate large quantities of scrap that cannot be used in the steel industry. Titanium scrap can also be employed in the electronics industry as a gettering agent. However the quantity of scrap that is used for this application is far exceeded by the quantity of turnings that are accumulated.
Recently U. S. Pat. No. 3,646,175 was issued to Bomberger et al for a method wherein machine turnings and other pieces of scrap could be reshaped into an intermediate product. This intermediate product is in the form of pellets or flakes which are thereafter blended with sponge metal. The blend can then be used to make up a pressed electrode for a melting charge.
The problems therefore confronting the prior art can be enumerated as follows:
Tin-bearing titanium scrap cannot be utilized as an alloying addition in the steel industry.
Vacuum arc re-melting and other commercial melting processes cannot consistently produce weldable, corrosion resistant forgeable stock from a scrap charge containing machine turnings because of high levels of carbon, nitrogen and oxygen.
Present commercial melting techniques cannot employ a 100% scrap charge and yield a usable product.
Titanium alloys in the form of machine turnings containing alpha stabilizers such as aluminum and tin and a high interstitial level including oxygen, carbon and nitrogen cannot be remelted and yield a usable product because of the poor physical properties of the resultant product.
Titanium machine turnings accumulate at a faster rate than they can be reclaimed and consumed.
The aforementioned problems are obviated by the present invention. A titanium alloy containing aluminum and tin as alpha stabilizers in the form of a 100% scrap charge with a high level of interstitials can be refined into a usable alloy.